Chain grab hooks grab a chain in such a way that the chain is prevented from running through the hook. To accomplish this end, the throat of the hook, defined by the shank portion at one side, the free-end portion at the other side, and an intermediate connecting bight portion, is made large enough to receive one link in an upright cross-over orientation but small enough to prevent passage therethrough of an adjacent link. The upright cross over link rests edgewise on the bridging bight portion at the lower end of the throat.
With presently constructed grab hooks, the chain, when placed under loaded conditions, is subjected to combined stresses, which, when large enough, ultimately cause the chain to fail. One of the stresses to which the chain is subjected is a tensile stress acting longitudinally of the chain. Chains are constructed to withstand such tensile stresses so that no problem is presented. When a chain link is seated in an upright or edgewise position in the throat of the grab hook to rest on the lower bight portion however, the cross-over link is also subjected to a shearing or cutting force. A chain link is not ordinarily designed to withstand such a shearing force acting transversely of the link. Accordingly, when a chain is used in conjunction with a grab hook, the breaking strength of the upright link within the throat of the hook is reduced so that, when overloaded, the chain invariably breaks at this point. It is therefore often necessary to use largr chains than are required to lift a desired load in order to assure an acceptable margin of safety. This, in turn, causes increased expense and inconvenience to the users of the chain.
In addition to possible chain failure, the hook is also subject to failure under either overload conditions or conditions of cyclic fatigue. Both the overload failure value and the fatigue life of a part are detrimentally effected by high stress concentrations as opposed to a more uniform overall stress distribution. A chain grab hook having a vertically oriented slot for receiving the cross-over link produces very high localized stress concentrations in the hook since the cross-over link is vertically supported by the hook in a very limited area of the saddle or bight portion of the hook. In many hook designs, the cross-over link, under the influence of the load, pivots at one side of the hook to lift off of the support saddle along most of its length with the result of being supported by an even smaller area of the hook. In order to prevent early hook failure, conventional hooks have of necessity been fabricated with excessive quantities of material and/or have been given a relatively low maximum load limit.
Accidental disengagement of the chain from conventional chain grab hook also presents a hazzard. In an unloaded or slack condition, it is possible for the cross-over link of the engaged chain to slide out of the engaging hook in a manner which either completely disengages the chain from the hook or removes the cross-over link to a position which would cause either chain or hook damage if the loaded condition were to be resumed.